Musnad Abu Ya'la al-Mawsili 42:307

Zuhayr narrated to us, Yazid bin Harun narrated to us, Muhammad bin ‘Amr informed us, from Abu Salamah, who said: We discussed the Night of Decree (Laylat al-Qadr), so we came to Abu Sa’id al-Khudri. I said to him, "O Abu Sa’id, did you hear the Messenger of Allah, may Allah bless him and grant him peace, mention the Night of Decree?" He said: The Messenger of Allah, may Allah bless him and grant him peace, performed I’tikaf during the middle ten days of the month of Ramadan, and we performed I’tikaf with him. When it was the morning of the twentieth, he returned and we returned with him. The Messenger of Allah, may Allah bless him and grant him peace, slept and saw the Night of Decree in a dream, then he was made to forget it. He went out in the evening and addressed us, saying: "I saw the Night of Decree in a dream, then I was made to forget it, and I was shown that night [in which] I prostrate in water and mud. So whoever performed I’tikaf with us, let him return to his place of I’tikaf. Seek it in the last ten nights, in the odd-numbered ones among them, for Allah is Odd and loves the odd." He said: So we returned, and the sky stirred upon us that evening, and the roof of the mosque was a trellis of palm branches. He performed I’tikaf, and by the One who honored him and revealed the Book to him, I saw it on the night of the twenty-first, and his forehead and the tip of his nose were in the water and the mud.